The present invention relates to a servo control system such as an industrial robot system, and particularly relates to the servo control system utilizing an observer for calculating a state value.
The servo control is adopted in various types of control systems which utilize a servo motor to control object actuator such as industrial robot, working machine, plotter and automatic assembler. Generally, the control system is constructed on the basic concepts of "Controllability" and "Observability". In order to effect the servo control, a pole of the controllable object is appropriately set by feeding back all of the state values. However, all of the state values may not be directly detected in some types of control systems. In constructing such control system, an observer can be utilized when the state value is observable. The observer can estimate the state value according to a detected signal so that the estimated state value is fed back to control the system.
In the conventional servo control system utilizing an observer, the control object is provisionally placed in a given origin position when starting the system. The observer calculates the state value with reference to the origin position.
On the other hand, when using a specific sensor such as an absolute type encoder having a function to detect an absolute position, a present stop position of the control object can be detected on restart of the control system. Therefore, in view of the reduction in operating time, it might be desirable to drive the control object to a target position directly from the stop position without returning the control object to the origin position. However, as described above, the conventional observer is constructed to calculate the state value with reference to the state where the control object is placed in the origin position. Stated otherwise, the initial state value is always set to zero so as to calculate a subsequent state value. Therefore, in the conventional observer, there would be considerable difference in the state value calculation between the origin position reference and the stop position reference. If the object were controlled according to the calculation of the state value using the stop position reference, there would result a drawback that dynamic performance of the control system would be seriously unstabilized in the initial state. Specifically, the control object would be seriously vibrated through an actuator, or an excess torque would be applied to the control object, or, even worse, the control object would be displaced in an unexpected direction so as to collide with an adjacent mechanism.
Therefore, the conventional servo control system utilizing an observer has the drawback that the control object cannot be driven safely from a given stop position.